This invention relates generally to thick film resistors and more particularly to a novel construction of a thick film resistor.
Thick film resistors are well known and are commonly formed by applying a paste of a resistive material onto an insulating substrate; the applied resistive material is then dried and hardened by the application of heat. The magnitude of the resistance of such a resistor depends upon the thickness of the resistive material, the composition of the material, and upon the extent of the area of the substrate covered with the resistive material.
One application of thick film resistors is as battery feed resistors in battery feed circuits for telephones. In such an application a source of power (e.g. a battery) is connected to a balanced two-wire subscriber telephone line (sometimes referred to as a loop) via two battery feed resistors in order to provide current to the line. The two battery feed resistors should be closely matched to each other in order to maintain the balance of the line, and are preferably small in physical size. The battery feed resistors should also be able to limit current on extremely short loops to safe values; at the same time they must be capable of carrying large currents that may arise from fault conditions (e.g. current surges due to lightning).
The following U.S. patents describe some of the known thick film resistor designs and attention is directed to them. U.S. Pat. Nos. 4,293,839 dated Oct. 6, 1981 by E. Asada et al.; 4,140,817 dated Feb. 20, 1979 by J. F. Brown; 4,097,988 dated July 4, 1978 by F. Hauschild; 3,947,801 dated Mar. 30, 1976 by K. R. Bube; 3,889,223 dated June 10, 1975 by L. Sella et al.; 3,787,956 dated Jan. 29, 1974 by T. F. Cocca et al.; and 3,573,703 dated Apr. 6, 1971 by D. P. Burks et al.